The field of the invention is multi-beam electron beam lithography.
It is generally accepted in the mask industry that single beam electron beam mask writers will not be able to deliver the current density at high resolution to achieve exposure speeds required for products below 100 nm GR (Ground Rules). The usable current in probe forming systems is limited by stochastic Coulomb interactions, primarily at apertures or beam crossovers, which translates into loss of resolution. By contrast, multibeam systems suffer much less from this problem since the total current delivered to the target is spread over many beams, in most cases, each with its own apertures and crossovers.
Multibeam systems proposed to date have problems with manufacturing feasibility primarily because of unattainable stability and uniformity requirements placed on the electron sources, i.e. field emitters and photocathodes. Some of these systems employ multibeams through part of the optics column but share the same crossover, which does not improve the electron interaction problem. Their stability requirements are further magnified since they typically use 1 to 1 imaging of the source at the target.
A uniform magnetic field (solenoid field) oriented along the electron beam axis is the simplest of electron lenses and has been employed in various electron beam systems. Electrons radiating from a point object execute, by virtue of their transverse velocity component, one cyclotron orbit in the transverse plane, returning to the optic axis. Thus, an image is formed with unity magnification. A major advantage of the solenoid lens is that there is no prescribed optic axis, hence a shift (deflection) of the beam by a transverse field will cause the beam to shift position, but maintain the same focal plane. A major disadvantage of these lenses is that they produce no demagnification of the object, so that defects in the source (reticle, shaping aperture) are reproduced in the image at the same scale.
The restriction of lenses formed by solenoid fields in the prior art to a one-to-one object to image ratio imposes severe limitations on the image quality. It is well known that the conventional object to image ratio of 4:1 in optical steppers is more xe2x80x9cforgivingxe2x80x9d, than a 1:1 demagnification ratio.
Such a 1:1 magnification ratio in a multiple-beam system is illustrated in U.S. Pat. Nos. 6,175,122, 5,981,962 and 5,962,859, which show a plurality of shaped-beam systems, contained within the same solenoid field. In such a system, imperfections in the aperture result in the same imperfections in the image, thus limiting resolution.
The invention relates to a multi-beam lithography system in which a set of electron beam sub-systems having a substantial demagnification are immersed in a solenoid field and operate in parallel.
A feature of the invention is the use of a single solenoid field common to several electron beam sub-systems.
Another feature of the invention is the use of magnetic lenses having substantial demagnification, so that imperfections in the object are reduced in the image by the demagnification factor.